1. Technical Field:
The present invention relates generally to protective shelters, and more particularly to redeployable mobile aboveground shelters with protected anchoring.
2. Description of the Related Art:
The construction of storm shelters, safe rooms and blast resistant modules is well known and thoroughly documented, for example, in FEMA 320, Third Edition and FEMA 361, Second Edition, both available from the Federal Emergency Management Agency (FEMA), as well as in ICC/NSSA 2008 “Standard for the Design and Construction of Storm Shelters,” published jointly by the International Code Council (ICC) and the National Storm Shelter Association (NSSA) and in Section 6, Wind Loads, of “Minimum Design Loads for Buildings and Other Structures,” SEI/ASCE 7-05, 2005, ISBN: 0-7844-0809-2, published by the American Society of Civil Engineers.
To meet safety standards, conventional shelters require either burial below ground or securement of an aboveground shelter to the ground in a manner that will afford the desired personnel protection. One challenge in protecting personnel located where severe wind events and other environmental hazards may occur is that the personnel are often stationed in such locations only temporarily and then moved to other locations. For example, crews working on drilling rigs, pipeline construction, wind turbine erection, petroleum refineries, compressor station repair, and road construction and repair are examples of frequently moved personnel that benefit from the protection provided by shelters. In many cases, use of below ground shelters is not practical, as they cannot easily, quickly and inexpensively be relocated to different work sites as work crews relocate.
Consequently, in many cases, it would be desirable to protect crews using aboveground shelters. One common aboveground shelter design fastens the shelter by numerous metal bolts or adhesives to a heavy foundation or concrete “pad”. For such pad-anchored aboveground shelters, the combined weight of the shelter plus its foundation or pad is often the primary factor relied upon to resist movement of the shelter (and thus provide protection of its occupants) during high velocity wind events. To a lesser degree, the large width of the required concrete foundation also helps the assembly resist overturning. Although pre-cast concrete community and industrial shelters are available, their immense weight (approximately 75,000 lbs. or more) requires the use of specially permitted and oversized trucks to haul them and heavy cranes to lift them into place, which renders their temporary redeployment impractical. Some conventional metal shelters can be unbolted from their heavy concrete foundations and moved more easily. However, each new location requires the preparation of another heavy concrete pad to which the shelter can be bolted. In most instances the cost and inconvenience of pouring a new pad (and the attendant environmental impact of the pad's subsequent demolition and removal) renders impracticable the redeployment of a pad-anchored protective shelter for temporary use.
A second type of aboveground shelter is an “anchored box” design that utilizes one or more exposed anchoring assemblies, including wire rope, steel cable, chains, turnbuckles, webbing straps and/or other type of securing cables to provide stability in high wind loads to a lightweight enclosure. FIG. 1 illustrates a typical installation of an aboveground shelter 100 employing an anchored box design. In this case, aboveground shelter 100 includes a metal or concrete enclosure 102 that is tethered to the underlying substrate 104 (e.g., the earth's surface) via multiple (in this example, four) anchoring assemblies 106. In various implementations, anchoring assemblies 106 are looped over and/or attached to enclosure 102 (e.g., via attachment rings 108) and also anchored to substrate 104 using any of a variety of anchoring devices 110, such as helical earth screws, driven piles, or bored holes filled with cement fitted with “eyes.” In the illustrated example, each anchoring assembly 106 includes a non-rigid component 112 (wire rope, steel cable or webbing strap) and a turnbuckle 114 that can be used to tension the anchoring assembly 106 and thus increase the effective weight of metal enclosure 102 and its resistance to movement in high winds.
It should be noted that in a typical installation, distance D1, which represents the effective height of the attachment point of anchoring assemblies 106 to metal enclosure 102, is approximately equal to distance D2, which is the distance along substrate 104 between the anchoring devices and the base of enclosure 102. However, given the variability of different installation locations and different installers, the angle A formed between anchoring assemblies 106 and substrate 104 commonly varies between 15 degrees to 60 degrees, with an angle of 45 degrees commonly considered to be optimal for resistance to wind-induced overturning, uplift and sliding forces on enclosure 102.